This disclosure relates to test and measurement instruments and, more particularly, to calibration of test and measurement instruments including one or more asynchronous time-interleaved digitizers, which use harmonic mixing for reducing noise.
Useable bandwidths of test and measurement instruments, such as digital oscilloscopes, can be limited by an analog to digital converter (ADC) used to digitize input signals. The useable bandwidth of an ADC can be limited to the lesser of the analog bandwidth or one half of a maximum sample rate of the ADC. Various techniques have been developed to digitize higher bandwidth signals with existing ADCs. One such technique is described in the above-referenced patent and applications, which includes splitting an input signal into a number of split signals each including substantially all of the bandwidth of the input signal. Then the split signals are respectively mixed with harmonic mixers and digitized. The digitized, split signals can be recombined to make a reconstructed input signal. This technique is referred to as ATI, or an Asynchronous Time Interleaved system.
In the event of interleaving errors due to analog mismatch of such a system, hardware adjustments can be made for mixing clock amplitude and phase. The adjustments can also be calibrated to minimize interleave mismatch spurs. Alternatively, or in addition, hardware mismatches can be characterized, and a linear, time-varying correction filter may be used to cancel the interleave spurs.
Previously, such calibration occurred at the factory before an instrument is shipped to a customer. Although the instruments are initially factory calibrated, hardware performance may drift from their calibrated state based on environmental conditions at runtime, such as temperature and humidity. Calibrating for a particular hardware state of such a sensitive device, however, requires access to a signal source that spans the full frequency range of the internal digitizer. The built-in calibration oscillators described in the '373 application, however, are not tunable over the entire range of the potential signal sources. Therefore, calibration of systems having a built-in calibration oscillator that does not span the entire range of potential signal sources suffers.
Embodiments of the invention address these and other limitations.